1. Field of the Invention
The present invention is directed to a method for detecting ventricular fibrillation by evaluating a measured impedance signal dependent on the blood volume in the heart, as well as to an apparatus for detecting and treating ventricular fibrillation.
2. Description of the Prior Art
Ventricular fibrillation is one of a number of rhythm disturbances of the heart exhibiting an unnaturally elevated frequency (contraction repetition rate) caused by pathologies of the heart's natural pacemaker, or pathologies arising in the conductive paths within the heart. Such rhythm disturbances are generically referred to as tachyarrhythmia, and result in a reduced ventricular filling and a reduced ejection power of the heart. In the worse case, i.e., ventricular fibrillation, the disturbances can result in a standstill of the blood conveying capability of the heart. In order to be able to administer a therapy technique best suited for treating the particular types of tachyarrhythmia, such as antitachycardiac stimulation, cardioversion or defibrillation, a recognition of the particular rhythm disturbance of the heart is first required. The detection of tachyarrhythmia only with the assistance of an intracardial electrogram and subsequent evaluation of criteria related to heartbeat rate, such as frequency, sudden frequency increase, or a persisting, high-frequency value, can thus be problematical.
In general, it is known to obtain a measured impedance signal which is dependent on the blood volume in the heart, and to evaluate the measured impedance signal to obtain information about the current condition of the heart. It is also known to activate and control pulse generating means for delivering electrical stimulation pulses to the heart based on the result of the evaluation of the impedance signal.
European Application 0 009 255 discloses the detection of ventricular fibrillation by evaluating both electrical and mechanical activity of the heart. Defibrillation of the heart is then triggered following the detection ventricular fibrillation. For acquiring signals corresponding to the electrical activity of the heart, the intracardial electrogram is obtained from the heart with an electrode arrangement. The mechanical activity of the heart is identified by obtaining a measured impedance signal, which dependent on the blood volume in the heart, using the same electrode arrangement, having an impedance measuring means connected thereto. Changes in the measured impedance signal which are dependent on the contractions of the heart are then evaluated.
In order to be able to identify the changes of the measured impedance signal which are dependent on the contractions of the heart in this known system, a plurality of individual measured values must be acquired and evaluated. The number of measured values required for this purpose is high in comparison to the contraction rate of the heart.